If she had mood swings before...I'd stay away from the Mirena...since it's the same hormones. More localized, yes, but anyone who reacts to oral pills can have similar reactions.
The benefit for a Mirena is that for MOST people they shorten or eliminate periods all together. Not true for everyone, some people get the opposite effect. Some people experience weight loss, some weight gain, some stay the same...pretty equal in groupings. Same problem as the pill that most people take a year+ to conceive after removal. Takes some time to become effective. Doesn't work when someone is on antibiotics, and then takes time to become effective again. Good for up to 5 years. NOT to be used by anyone with a history of abnormal paps/cervical cancer (the hormones can make it worse/more aggressive).
I've had the copper iud (paraguard) for 7 years now...and am in the 'looove it' category. Benefits of the Paraguard; no hormones, good for 10 years, is effective the minute it is placed and is effective while on antibiotics...no waiting period ever. Downsides...first couple periods are gnarly (2-3 typically). Usually no change in menstral flow/cramping after that...although a small percentage go lighter and a small percentage go heavier. No associated mood/weight changes (no hormones). NOT to be used by anyone allergic to copper. ;)
BTW...last person I know who got one at planned parenthood (Paraguard) got hers last year for around 150...I had mine done by my OB (covered 100% by insurance) seven years ago. If I'd paid out of pocket, it would have been 750. Anyone on Pregnancy Medical through the state can get one for free. I have 3 years to go, but as of writing it's still covered at 100% by our insurance. I have no clue the out of pocket expense for Mirena (more, less, or the same), or out of pocket through an OB for Paraguard. Anyone?